MALDI Mass Analysis of 11 kDa Gold Clusters Protected by

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MALDI Mass Analysis of 11 kDa Gold Clusters Protected by Octadecanethiolate Ligands† Risako Tsunoyama,‡ Hironori Tsunoyama,‡ Panvika Pannopard,§ Jumras Limtrakul,§ and Tatsuya Tsukuda*,‡ Catalysis Research Center, Hokkaido UniVersity, Nishi 10, Kita 21, Sapporo 001-0021, Japan, and Department of Chemistry and Center of Nanotechnology, Kasetsart UniVersity, Bangkok 10900, Thailand ReceiVed: February 26, 2010; ReVised Manuscript ReceiVed: July 8, 2010

We previously reported the isolation of octadecanethiolate-protected Au (Au:SC18H37) clusters having a core mass of ∼11 kDa from the crude Au:SC18H37 samples obtained in the reaction between octadecanethiol (C18H37SH) and poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Au clusters. This stable Au:SC18H37 compound was assigned to be Au55(SCnH2n+1)32 by destructive mass spectrometry and thermogravimetry (TG) performed in the framework of the classical structure model, in which the thiolates are bound to the surface of a magic Au55 core. In the present study, the molecular formula of the Au:SC18H37 (11 kDa) cluster was reassessed by non-destructive matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS), and it was confirmed that the Au:SC18H37 (11 kDa) cluster is a mixture of Au54(SC18H37)30 and Au55(SC18H37)31. On the basis of our present understanding of the structures of other stable Au:SR clusters, we proposed that Au54(SC18H37)30 and Au55(SC18H37)31 have a structural motif comprising a Au37 cluster core that is completely protected by -SR-[Au(I)-SR-]x oligomers (x ) 1 and 2). The nonformation of the Au:SR (11 kDa) cluster during the conventional chemical reduction of Au(I)-SR oligomers is ascribed to the kinetics of Au:SR formation. Introduction In 1994, Schiffrin developed a simple method involving the reduction of Au(I)-SR oligomers1 to prepare Au clusters whose surfaces were chemically passivated by thiolate monolayers (Au: SR) (Figure 1A). Shortly after this report, Whetten2 and Murray3,4 synthesized small, monodispersed Au:SR clusters (core diameter < 2 nm), whose electronic and optical properties were dependent on the core size and significantly different from those of the bulk Au. Whetten isolated a series of stable Au: SR clusters by treating them as normal chemical entities and evaluated their core masses to be ∼8, 14, 22, and 29 kDa using laser desorption ionization mass spectrometry (LDI-MS).5–7 Later, we employed the nondestructive electrospray ionization MS (ESI-MS) technique and demonstrated that the Au:SR clusters with well-defined molecular formulas can be isolated.8–10 We also showed that in the Schiffrin method, metastable Au: SR clusters are produced concomitantly along with highly stable Au:SR clusters because of kinetic hindrance during the growth of the Au core;11 further, we showed that the stable Au:SR clusters can be selectively formed by core etching using thiols.12 Au25(SR)18 is a prototypical system that has been discovered in such a manner.8,13,14 Now, the size focusing of preformed Au: SR clusters by thiol etching has become a general protocol for selective synthesis of magic Au:SR clusters.7,15–18 Recent experimental efforts have enabled researchers to share a series of highly stable Au:SR clusters such as Au20(SR)16,19 Au25(SR)18,20–22 Au38(SR)24,15,16 Au44(SR)28,17 Au68(SR)34,23 Au102(SR)44,24 and Au144(SR)60.18,25 † Part of the special issue “Protected Metallic Clusters, Quantum Wells and Metallic Nanocrystal Molecules”. * To whom correspondence should be addressed. E-mail address: [email protected]. Fax: +81-11-706-9156. ‡ Hokkaido University. § Kasetsart University.

As for their geometrical structures, it had long been believed that the thiolates are bound to the bridged or hollow sites on the facets of the Au nanocrystals26,27 based on the time-honored structure model for the self-assembled monolayer (SAM) of thiolates on an extended Au surface.28,29 Garzo´n was the first to point out that thiolate ligation can cause significant structural distortion of the Au core.30,31 Ha¨kkinen extended this idea to propose a “divide-and-protect” concept, according to which thiolates form cyclic tetramers, [Au(I)-SR-]4, on the surface of the Au core.32 Nobusada proposed a “core-in-cage” model, in which the Au core is caged by the larger cyclic [Au-SR-]12 oligomers.33 In 2007, Kornberg24 achieved a breakthrough in the research of Au:SR systems by elucidating the structure of Au102(SR)44 using single-crystal X-ray diffraction. Subsequently, Murray20 and Jin21 experimentally determined the structure of Au25(SR)18, while Ha¨kkinen34 theoretically predicted the structure of this cluster. These reports have dramatically changed the traditional view of the structure of Au:SR clusters;35,36 Au102(SR)44 and Au25(SR)18 have a common structural motif and are composed of a highly symmetrical Au core whose surface is protected by -SR-[Au-SR-] and/or -SR[Au-SR-]2 bidentate ligands. The stabilities of Au102(SR)44 and Au25(SR)18 have been explained in the theoretical studies performed by Aikens,21 Li,37 and Han.38 Recent theoretical studies have predicted that other stable Au:SR clusters such as Au20(SR)16,39,40 Au38(SR)24,41–43 Au44(SR)28,44 and Au144(SR)6045 have a similar structural motif. The theoretical prediction made for Au38(SR)2442,43 was confirmed by Jin, who carried out singlecrystal X-ray crystallography studies; the results revealed that Au38(SR)24 is composed of a face-fused bi-icosahedral Au23 core capped by three -SR-[Au-SR-] and six -SR-[Au-SR-]2 ligands.46 Electronic structure is another determining factor of the stability of Au:SR clusters. The high stability of specific charge states such as [Au25(SR)18]-1 (refs 9 and 20–22), [Au44(SR)28]-2 (ref 17), [Au68(SR)34]0 (ref 23), and [Au102-

10.1021/jp101741a  2010 American Chemical Society Published on Web 07/23/2010

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Figure 1. Preparation of Au:SR cluster via (A) chemical reduction of the Au(I)-SR oligomer and (B) thiolation of polymer-stabilized Au(0) clusters.

(SR)44]0 (ref 24) is explained by the electron counting rule (superatom concept);47 however, the charge state of Au:SR clusters can be altered depending on the synthesis and storage conditions and by redox reactions.9,48,49 Another frequently used method for preparing Au:SR clusters involves replacement of the ligands on the preformed Au(0) clusters with thiolates.50 The reaction of thiols with Au(0) clusters that are weakly stabilized by a polymer chain can be viewed as the SAM formation on the three-dimensional Au surface (Figure 1B). In 2006, we reported that reaction of n-alkanethiol CnH2n+1SH (n ) 12 and 18) with small Au clusters stabilized by poly(N-vinyl-2-pyrrolidone) (PVP) yields stable Au:SCnH2n+1 clusters with unprecedented Au core masses of ∼11 and ∼26 kDa.51,52 The high stability of these Au:SCnH2n+1 clusters (core mass: 11 and 26 kDa) is mainly governed by the structure of the Au and S moieties since van der Waals interactions between the alkyl chains are negligibly small. In our 2006 study, we tentatively assigned the 11 kDa Au:SCnH2n+1 cluster to Au55(SCnH2n+1)32 on the basis of the results of destructive LDI-MS and thermogravimetry (TG)51 performed in the framework of the classical structure model, in which the thiolates are bound to the surface of the “magic” Au55 core, as in the case of the well-known Au cluster compound Au55(PR3)12Cl6.53 The monodentate and bidentate ligation of thiolates to the Au clusters has been demonstrated by crystallographic studies of Au11(SR)3(PPh3)7 (ref 54) and [Au25(SR)5(PPh3)10Cl2]2+ (ref 55), respectively. The aims of the present study are as follows: (1) determination of the molecular formula of the 11 kDa Au:SC18H37 cluster by nondestructive MS, (2) reinterpretation of the structure of this compound according to the current understanding, and (3) identification of the reason for the preferential formation of this compound in the thiolation reaction. Experimental Section

an aqueous solution (5 mL) of HAuCl4 (0.05 mmol) and PVP (0.5 mmol of monomer units) was mixed with an aqueous solution (5 mL) of NaBH4 (0.25 mmol) and PVP (0.5 mmol) in a microfluid reactor maintained at 313 K.56 The brown hydrosol eluted from the mixer was stirred for 1 h at 273 K. Then, a toluene solution (25 mL) of C18H37SH (0.25 mol) was placed on top of the Au:PVP hydrosol, and the resulting biphasic mixture was vigorously stirred at room temperature to obtain a uniform emulsion. After some time, the aqueous phase became colorless, and the toluene phase became brown, confirming the transformation of the protecting layer from PVP to C18H37S. After 2 h of mixing, the aqueous phase was discarded, and the organic phase was evacuated to dryness. The crude Au:SC18H37 clusters were washed with ethanol and incubated in neat C18H37SH (2 mL) at 353 K for 20 h with stirring. The resulting Au:SC18H37 clusters were washed three times with warm ethanol and pretreated on a silica column. Then, the product was fractionated by size using a recycling size-exclusion chromatography (SEC) system (LC-908, Japan Analytical Industry Co., Ltd.) equipped with two columns (JAIGEL-W253, Japan Analytical Industry Co., Ltd.) connected in series.51,52 Toluene was used as an eluent (flow rate: 3.5 mL/min), and the eluted clusters were optically detected at 290 nm. UV-vis/near-IR absorption spectra of the fractionated Au: SC18H37 clusters in toluene were measured under ambient conditions by using a spectrophotometer (V670, JASCO). The absorbance in wavelength units, I(w), were converted to that in energy units, I(E), by using the relation I(E) ∝ I(w) · w2.57 For mass analysis, the Au:SC18H37 samples were mixed with trans2-[3-(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile (DCTB)15,18,22,23,58,59 in a typical molar ratio of 1:500 and irradiated by a N2 laser (337 nm). Matrix-assisted laser desorption/ionization (MALDI) mass spectra were recorded using a linear time-of-flight (TOF) mass spectrometer (VoyagerDE STR-H, Applied Biosystems) in the positive-ion mode.

n-Alkanethiol CnH2n+1SH (n ) 12, 16, and 18) was used as the protecting ligand to study the intrinsic stability of the 11 kDa Au:SR clusters. In the present study, we focused on octadecanethiolates (SC18H37) because the yield of the 11 kDa Au clusters was highest. The 11 kDa Au:SC18H37 clusters were synthesized in the following two steps: (1) preparation of Au clusters stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP) and (2) reaction of the Au:PVP clusters with C18H37SH.51,52 First,

Results and Discussion 1. Synthesis of the 11 kDa Au:SC18H37 Cluster. The sizeexclusion chromatogram of the crude Au:SC18H37 sample originally showed a single band, which split into two bands upon continuous recycling. Figure 2A shows the presence of two bands peaked at 225 and 231 min in the chromatogram obtained in the sixth cycle. In this cycle, eluent fractions were

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Figure 2. (A) Chromatogram of crude Au:SC18H37 sample in the sixth cycle. (B) Optical spectra of fractions 1 and 2. The dotted line represents the baseline of the spectrum of 2.

Figure 3. Positive-ion MALDI mass spectra of fraction 1 recorded at low (a) and high (b) laser power. Asterisks indicate the parent and fragment ions of the Au38(SC18H37)24 impurity. Details of the assignment are provided in Figure S1 (Supporting Information).

collected at 1 min intervals and characterized by optical spectroscopy. Figure 2B shows the absorption spectra of fractions 1 and 2. The spectral profile of fraction 1 was very similar to that reported previously for Au:SC18H37 clusters with a core mass of ∼11 kDa.51,52 The purity of the present sample was higher than that of the previous studies51,52 judging from the well-structured spectral profile. The optical gap was determined to be ∼0.9 eV.60 The optical spectrum of fraction 2, on the other hand, was similar to that of Au38(SC18H37)24.10,18 Thus, we concluded that the major components of fractions 1 and 2 were Au:SC18H37 clusters (core mass: ∼11 kDa) and Au38(SC18H37)24, respectively. These clusters showed no sign of degradation as long as they were stored in powder form in a refrigerator. 2. Molecular Formula of the 11 kDa Au:SC18H37 Cluster (Fraction 1). For determining the molecular formula of the 11 kDa Au:SC18H37 cluster in fraction 1, it is essential to suppress the fragmentation of the clusters so that it is ionized in the intact form. Figure 3 shows the typical MALDI mass spectra of fraction 1 recorded at different laser fluences. Spectrum (b), which was recorded at a high laser power, shows mass peaks assignable to [AunSm]+, as observed in the conventional LDI mass analysis. The mass spectral pattern changed significantly with a decrease in the laser fluence; spectrum (a) is a typical MALDI mass spectrum recorded at the minimum laser fluence required for ion detection. The extremely low efficiency for the

Tsunoyama et al.

Figure 4. MALDI mass spectra of Au38(SC18H37)24 (fraction 2) recorded in the positive-ion mode. Laser power increases in the order (a) < (b) < (c). The inset in panel (b) shows the splitting of the [Au38Sm]+ peaks.

desorption and ionization of the 11 kDa Au:SC18H37 cluster gave rise to two problems in the analysis of spectrum (a). The first problem was the contamination of the spectrum by peaks due to an impurity. To our surprise, we found that the dominant mass peaks observed in the m/z range of 11500-14500 Da were attributed to the impurity Au38(SC18H37)24 (Figure S1, Supporting Information), whose concentration in fraction 1 was extremely small, as was evident from the optical spectra shown in Figure 2B. This result indicated that the ionization efficiency for the 11 kDa Au:SC18H37 cluster was considerably lower than that for the Au38(SC18H37)24. The second problem was the trade-off between the ion intensity and the degree of laser-induced fragmentation. We observed a series of weak peaks in the m/z range of 16000-20000 Da, where peaks due to the parent ions generated from the intact 11 kDa Au:SC18H37 cluster were expected to appear. Despite extensive efforts, fragmentation from the 11 kDa Au:SC18H37 cluster could not be suppressed even at the minimum laser fluence. Hence, it is important to distinguish the parent species from the fragment species for the assignment of the clusters in fraction 1 to a specific molecular formula. For accurate assignment of the peaks in spectrum (a) (Figure 3), we first studied the fragmentation pattern of a known cluster compound Au38(SC18H37)24 present in fraction 2. Figure 4 shows the MALDI mass spectra of Au38(SC18H37)24 (fraction 2) as a function of laser power; the laser power increases in the order (a) < (b) < (c). When the laser power is just above the ion detection threshold, the peak due to the intact parent ion [Au38(SC18H37)24]+ is detected along with several fragment ion peaks (spectrum (a)). The major fragmentation channels include the loss of Au4(SC18H37)4, Au4(SC18H37)4 + C18H37, and the SC18H37 ligands (see also Figure 5). Previous studies have reported the fragmentation of the Au4(SR)4 unit in the MS analysis of Au38(SC2H4Ph)24 (ref 15) and Au68(SC2H4Ph)34 (ref 23). The preferential loss of Au4(SC18H37)4 can be attributed to the high stability of Au4(SC18H37)4 in the ring form.61 With an increase in the laser power, the peak due to the intact parent ion disappears, while new peaks appear (spectrum (b)). The most dominant peaks are assigned to gold sulfide clusters [Au38S13-17]+ and [Au34S12-15]+, while the minor peaks observed in the m/z range of 8000-12000 Da contain several alkyl groups (Figure S2, Supporting Information). It is important to note that

MALDI Mass Analysis of 11 kDa Gold Clusters

J. Phys. Chem. C, Vol. 114, No. 38, 2010 16007 arrows can be assigned to the parent ions whose molecular formulas are Au54(SC18H37)30 and Au55(SC18H37)31, respectively.62 These molecular formulas are slightly different from the actual formula determined on the basis of our original assignment of Au55(SC18H37)32;51 however, we believe that Au54(SC18H37)30 and Au55(SC18H37)31 are not the products of the selective photofragmentation of Au55(SC18H37)32. The assignment of the two aforementioned parent ions is confirmed by the MALDI-MS analysis of the 11 kDa Au cluster protected by C16H33S (ref 63). The assignment is further supported by the fact that the fragmentation pattern of Au54(SC18H37)30 and Au55(SC18H37)31 is similar to that of Au38(SC18H37)24. Figure 5 compares the detailed fragmentation patterns obtained for Au38(SC18H37)24, Au54(SC18H37)30, and Au55(SC18H37)31; the mass peaks corresponding to Au38(SC18H37)24 and Au55(SC18H37)31 are aligned vertically for ease of comparison. As can be seen from the figure, the three clusters show a similar fragmentation pattern, that is, loss of several SC18H37 ligands, Au4(SC18H37)4, and Au4(SC18H37)4 + C18H37. Thus, we conclude that the 11 kDa Au:SC18H37 clusters are composed of Au55(SC18H37)31 and a measurable amount of Au54(SC18H37)30. 3. Structure Model of Au54(SC18H37)30 and Au55(SC18H37)31. The high stability of Au54(SC18H37)30 and Au55(SC18H37)31 can be ascribed to their electronic and geometric structures. However, it is difficult to explain the reason for this high stability on the basis of the electronic structures of the clusters alone since the electronic charge state of the clusters is not clear. Hence, we focus on the geometric structures of Au54(SC18H37)30 and Au55(SC18H37)31. One may expect the structure of the interface between the Au(0) cores and the thiolates in Au54(SC18H37)30 and Au55(SC18H37)31 to be different from the well-accepted -SR-[Au-SR-]x oligomer structure; this is because the aforementioned clusters are obtained only by the thiolation of Au:PVP and not by the reduction of the Au(I)SR oligomer (Figure 1). In our previous study,51 we speculated that 32 thiolates are bound to the facet of the Au55 core having an icosahedral or cuboctahedral motif, similar to the case of Au55(PR3)12Cl6, which has a cuboctahedral Au55 core.53 A recent computational study performed at the density functional theory (DFT) level on Au55(SCH3)32 revealed that this cluster has a local minimum structure in which a deformed icosahedral Au55 core is coordinated with 32 RS ligands via Au-SR single bonds.64 However, with the Au core-RS shell structure model, we cannot explain why Au54(SC18H37)30 and Au55(SC18H37)31 prefer the Au54 core and 31 thiolate ligands, respectively. The following discussions lead us to conclude that Au54(SC18H37)30 and Au55(SC18H37)31 can be categorized as Au:SR clusters having a -SR-[Au-SR-]x shell around the Au core. Table 1 lists all possible combinations of the Au core size and the number of the -SR-[Au-SR-]x (x ) 1 and 2) oligomers. Let us consider the most reasonable combination among these.

Figure 5. Comparison of the fragmentation patterns of (a) Au38(SC18H37)24 and (b) Au54(SC18H37)30 with Au55(SC18H37)31. Mass peaks of Au38(SC18H37)24 and Au55(SC18H37)31 are aligned vertically for ease of comparison.

the number of Au atoms in the larger fragment [Au38Sm]+ is identical with that of that parent Au38(SC18H37)24, regardless of the extensive fragmentation in the MALDI process. Further increase in the laser power results in the formation of a series of [AunSm]+ ions with ∼30 < n < ∼50 (spectrum (c)); this is because of the laser-induced fragmentation and aggregation of the Au38 core. This observation indicates that the MALDI mass spectra do not provide direct information on the number of Au atoms when the laser power exceeds a certain critical value. The characteristic fragmentation of Au38(SC18H37)24 (spectrum (b) in Figure 4) prompted us to determine the number of Au atoms present in the clusters in fraction 1 on the basis of the mass spectra recorded at a high laser power. Spectrum (b) in Figure 3, which is recorded at a high laser power, shows a mass peak assignable to [AunSm]+. Further, this spectrum includes two sets of doublet peaks at n ) 55/51 and 54/50 with a spacing equal to the mass of the Au4 unit, as in the case of Au38(SC18H37)24. The doublet peaks at n ) 54 and 50 cannot be assigned to the fragments of [Au55Sm]+ and [Au51Sm]+, respectively, because the peaks were observed even when the laser power was decreased to a considerable extent. These spectral features lead us to conclude that fraction 1 contains two AuN(SC18H37)M clusters, one with N ) 54 and the other with N ) 55. Then, we attempted to assign the peaks in spectrum (a) shown in Figure 3 by assuming that the number of Au atoms is either 54 or 55. The mass peaks indicated by blue and red

TABLE 1: Structural Models for Au54(SC18H37)30 and Au55(SC18H37)31 numbers of -SR-[Au-SR-]x oligomersa Au54(SR)30a

Au55(SR)31a

Au core size

x)1

x)2

x)1

x)2

total numbers of S anchors

outer/inner atoms in the Au core

39 38 37 36 35 34

15 12 9 6 3 0

0 2 4 6 8 10

14 11 8 5 2

1 3 5 7 9

30 28 26 24 22 20

30/9 28/10 26/11 24/12 22/13 20/14

a

RS represents C18H37S.

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Tsunoyama et al. is similar to that considered in the production of the ubiquitous magic cluster Au38(SR)24; in the Schiffrin method, the direct yield of Au38(SC18H37)24 is very low.51,52 The yield of Au38(SR)24 can be increased significantly by thiol etching of larger Au:SR clusters.15,16,51,52,67 The nonformation of Au38(SR)24 and the Au: SR (11 kDa) cluster during the Schiffrin method is ascribed to the different kinetics of Au:SR formation. Summary

Figure 6. Population of Au atoms in the core and -SR-[Au-SR-]x oligomers in the AuN(SR)M compounds reported thus far. The inset shows the theoretical42,44,45 and experimentally observed20,21,24,46 geometrical structures of the Au cores.

Previously, we had proposed three empirical rules that govern the formation of a stable Au:SR cluster10 on the basis of the structures of Au102(SR)44 (ref 24) and Au25(SR)18.20,21,34 First, the Au cores should form highly symmetric and stable geometric structures. Second, all of the surface atoms on the Au core should bind to both ends of the thiolates of -SR[Au-SR-]x. Third, the number ratio of -SR-[Au-SR-]2 to -SR-[Au-SR-]1 should decrease with an increase in the core size, probably because larger Au cores with a smaller degree of curvature prefer -SR-[Au-SR-]1 over SR-[Au-SR-]2. Figure 6 shows the population of the Au atoms in the core and the -SR-[Au-SR-]x layers as a function of the total numbers of Au atoms determined experimentally and theoretically.15–25,37–46 The populations of Au atoms evolve smoothly across Au54(SC18H37)30 and Au55(SC18H37)31 if we assume that their Au core size is 37. According to the empirical rule, the Au37 core should comprise 26 atoms in the outer and inner layer and 11 atoms in the inner layer.65 However, detailed experimental and theoretical studies should be carried out to reveal the exact structures of Au54(SC18H37)30 and Au55(SC18H37)31. The Au54(SC18H37)30 and Au55(SC18H37)31 clusters show a characteristic fragmentation pattern in the MALDI process, similar to Au:SR clusters having a -SR-[Au-SR-]x (x ) 1 and 2) interfacial structure, such as Au38(SC18H37)24 (Figure 4), Au38(SC2H4Ph)24,15 and Au68(SC2H4Ph)34;23 these observations support the results obtained using the aforementioned structure model. How is such an interfacial structure formed during the thiolation of Au:PVP clusters (Figure 1)? A recent MALDI study has revealed that the Au:PVP samples prepared under the conditions employed in the present study contain magic Au clusters (Au34, Au42, and Au58).66 The present results indicate that the thiols pull out the Au atoms from the magic Au clusters to afford a -SR-[Au-SR-]n interface; further, the thiol molecules etch out Au atoms from the Au clusters so that Au54(SC18H37)30 and Au55(SC18H37)31 are selectively populated with the desired Au core and the appropriate number of thiol ligands. Finally, we attempt to clarify why Au54(SC18H37)30 and Au55(SC18H37)31 cannot be produced by the Schiffrin method (Figure 1A), although they have a structural motif similar to that of the other members of the Au:SR family. The situation

First, Au:SC18H37 clusters were prepared by a reaction between C18H37SH and mixtures of PVP-stabilized Aun clusters (n ) 34, 42, 58). The Au:SC18H37 samples thus prepared were subjected to recycling SEC to afford a Au:SC18H37 cluster (core mass: 11 kDa) and Au38(SC18H37)24. On the basis of the MALDI mass spectra recorded at a low laser power, we concluded that the 11 kDa Au:SC18H37 cluster is a mixture of Au54(SC18H37)30 and Au55(SC18H37)31. The stability of these clusters could not be explained by the classical structure model, in which the thiolates are bound to the Au54 and Au55 cores. Alternatively, we propose that Au54(SC18H37)30 and Au55(SC18H37)31 are composed of a Au37 core whose surface is completely protected by -SR-[Au-SR-]x (x ) 1 and 2) oligomers. The fact that the 11 kDa Au:SR clusters were formed only via the thiolation of Au:PVP could be ascribed to the reaction kinetics rather than to the formation of new Au:SR clusters with different interfacial structures. Acknowledgment. This study was financially supported by a Grant-in-Aid (Grant No. 18064017, Synergy of Elements), MEXT Japan. This research was conducted under the JSPS Exchange Program for East Asian Young Researchers (JENESYS 2009). P.P. and J.L. thank the National Science and Technology Development Agency (NSTDA Chair Professor and National Nanotechnology Center) and the Thailand Research Fund for their support. The MALDI-MS analysis was carried out at the open facility in Hokkaido University. Supporting Information Available: Mass assignment of the peaks in Figures 3 and 4 and the structure model of the Au37 core. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (2) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1996, 8, 428. (3) Terrill, R. H.; Postlethwaite, T. A.; Chen, C. H.; Poon, C. D.; Terzis, A.; Chen, A. D.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M.; Johnson, C. S.; Samulski, E. T.; Murray, R. W. J. Am. Chem. Soc. 1995, 117, 12537. (4) Templeton, A. C.; Wuelfing, M. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (5) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L.; Cullen, W. G.; First, P. N.; Gutie´rrezWing, C.; Ascensio, J.; JoseYa´caman, M. J. J. Phys. Chem. B 1997, 101, 7885. (6) Chen, S. W.; Ingram, R. S.; Hostetler, M. J.; Pietron, J. J.; Murray, R. W.; Schaaff, T. G.; Khoury, J. T.; Alvarez, M. M.; Whetten, R. L. Science 1998, 280, 2098. (7) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785. (8) Negishi, Y.; Nobusada, K.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 5261. (9) Negishi, Y.; Chaki, N. K.; Shichibu, Y.; Whetten, R. L.; Tsukuda, T. J. Am. Chem. Soc. 2007, 129, 11322. (10) Chaki, N. K.; Negishi, Y.; Tsunoyama, H.; Shichibu, Y.; Tsukuda, T. J. Am. Chem. Soc. 2008, 130, 8608. (11) Negishi, Y.; Takasugi, Y.; Sato, S.; Yao, H.; Kimura, K.; Tsukuda, T. J. Phys. Chem. B 2006, 110, 12218.

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